This invention relates to workcenters of the type comprising a power tool mounted in a frame for movement in three dimensions under manual or automatic control.
Such workcenters are known, and usually comprise three linear movement tracks arranged along orthogonal axes. Each track may comprise an extruded section girder mounting a carriage moveable along the track in a first direction. The carriage may then carry a second girder, arranged in second, perpendicular, direction. It has its own carriage moveable in that direction and carries a third girder. The third girder is arranged in a third direction, perpendicular to the first and second directions, and carries a third carriage moveable in that direction. The third carriage carries the tool which is thus positionable at any point in space within the confines of the allowable movement of the carriages in their respective tracks.
How each carriage is mounted in each track is a matter of design choice, but to ensure sufficiently accurate movement of the tool carried on the third carriage, each carriage must only have freedom of movement along its respective axis and, moreover, its position along the track must be determinable with accuracy. Otherwise, the workcenter will not have sufficient accuracy to perform useful functions.
The present invention is concerned with the positioning of a carriage along its track. Several methods are available for driving a carriage. For example, a rack and pinion arrangement may be employed, or an endless toothed belt may be connected to the carriage and driven by a motor, or indeed a worm-drive screw may be rotated by a motor and passing through a correspondingly threaded nut on the carriage. The present invention is concerned with the latter arrangement.
A worm drive is a satisfactory arrangement because it can be driven directly by a stepping motor capable of rotating in many discrete steps per rotation resulting in very fine movements of the carriage. Hitherto, however, the problem with this arrangement is that to achieve the required accuracy, a very accurately machined worm drive screw of length equal to the maximum permissible movement of the carriage must be provided. Such a screw, and the nut which cooperates therewith, are together very expensive to manufacture. It is therefore an object of the present invention to provide an arrangement whereby a much less expensive rolled steeled screw can be employed without significant loss of accuracy. Such a screw has hitherto not been used because it is liable to dimensional inaccuracy which precludes a close fitting nut. U.S. Pat. No. 3,831,460 describes an anti-backlash nut which goes some way towards overcoming this problem but is not a complete solution.